1. Field of the Invention
The present invention relates to an electric circuit for supplying a constant voltage to loads connected thereto.
2. Description of Related Art
Examples of an electric circuit for supplying a constant voltage are disclosed in JP-A-2003-150256 and in JP-A-7-261862. Generally in the conventional circuits, a Zener diode for generating a reference voltage is used. An essence of the conventional circuits is shown in FIG. 6 attached hereto. Between a power source line 1 connected to a power source (such as a battery having voltage VB) and a ground 2, a series circuit composed of a constant current circuit 3 and a reference voltage supplying circuit 4 is connected. The reference voltage supplying circuit 4 is a series circuit consisting of a diode D1 and a Zener diode D2 connected in series. The reference voltage supplying circuit 4 supplies a reference voltage Vr to a base of a transistor Q1. A collector of the transistor Q1 is connected to a power source line 1, and an emitter thereof is connected to an output terminal 5.
The diode D1 connected to the Zener diode D2 in series functions to cancel a base-emitter voltage Vbe of the transistor Q1 with a forward voltage Vf of the diode D1. It is intended, in this manner, to decrease temperature dependency of the output voltage Vout and to control the output voltage Vout to a voltage substantially equal to a Zener voltage Vz. However, the base-emitter voltage Veb of the transistor Q1 becomes unequal to the forward voltage Vf of the diode V1 when an amount of output current changes, because the base-emitter voltage Vbe of the transistor Q1 changes depending on an amount of output current. This means that the base-emitter voltage Vbe of the transistor Q1 cannot be always canceled with the forward voltage Vf of the diode D1, and therefore the output voltage Vout cannot be maintained at a constant level with high accuracy.